Wavelength division multiplexers combine optical signals having different wavelengths and output a wavelength division multiplex signal (WDM-signal). Wavelength division demultiplexers divide a wavelength division multiplex signal into a plurality of optical signals having different wavelengths. Both, multiplexers and demultiplexers, are fixed wavelength band-pass filters. Transceivers (transmitters and receivers) are connected to terminal points of said multiplexers and demultiplexers. Each transmitter (part of the transceiver) emits and each receiver (part of the transceiver) receives an optical signal with a certain wavelength. Each transceiver is connected to certain ports of the terminal, which means that new interconnections are necessary if the number of transceivers is upgraded or new connections between ports and receivers are necessary. Several transceivers can be attached as front ends to an electrical switching matrix.